Interpretive Summary: Carbohydrates comprise about 70% of a dairy cow's diet, with 30 to 40% of diet dry matter provided by nonfiber carbohydrates (NFC). Commonly fed NFC sources are corn, citrus pulp, and molasses which contain starch, sugars + soluble fiber, and sugars, respectively, as their predominant NFC. These NFC tend to be digested mostly by microbes in the cow's rumen. Sufficient protein that can be degraded and used by rumen microbes is essential to the efficient digestion of carbohydrates in the rumen and provision of nutrients to the cow to support milk production. However, we do not know how the protein requirements of the cow or microbes change with different NFC types, and how that affects cow and microbial performance. In a study with lactating dairy cows, we tested the effect of feeding more or less degradable protein from soybeans with feeding NFC from corn (starch), citrus pulp (sugar+soluble fiber), and sucrose+molasses (sugar). We found that responses in milk production, efficiency of feed utilization, fiber digestion, and ruminal values that reflected differences in fermentation by the microbes were affected by NFC type, protein degradability, or the interaction of the two. This study showed that it is important to consider the type of NFC and amount of degradable protein fed when formulating diets to best meet the nutrient requirements of cattle to enhance production and efficiency of feed utilization. Application of these concepts can in turn enhance farm profitability and reduce the amounts of nutrients excreted into the environment.

Technical Abstract:
Effects of nonfiber carbohydrate source (NFC) and protein degradability (RDP) in the diets of lactating dairy cattle on intake, production, efficiency, and ruminal measures were evaluated in a three period (21 d) partially balanced incomplete latin square design with a 3x2 factorial arrangement of treatments. Dietary treatments included three NFC sources (ground corn=starch=ST; molasses+sucrose=sugar=SU; and citrus pulp=soluble fiber+sugar=SF) and two concentrations of ruminally degradable protein (+or-RUP) achieved by addition or omission of expeller soybean meal. Total mixed rations were formulated to be isonitrogenous and offered ad libitum. Dry matter intake was affected by NFC source, with cows fed SU having a greater intake than those fed SF. Milk and milkfat production as well as efficiencies of fat and protein corrected milk production and dietary nitrogen utilization were affected by the interaction of NFC x RDP, with the production of cows fed ST increasing with more RDP, and animals fed SU or SF declining or remaining the same with increased RDP in the diet; there was a tendency for milk protein production to show this same effect. Cows fed diet ST or those receiving more RDP had the greatest milk protein production and milk urea nitrogen concentrations. Ruminal concentrations of acetate, propionate, and butyrate did not differ among treatments, but molar % of acetate were greater on SF than SU, and tended to decrease with greater RDP. Propionate molar % was greater at greater RDP. Molar % of butyrate was lowest with ST, compared to SU and SF. Ruminal ammonia tended to be greatest with ST, though unaffected by RDP. Branch chain organic acids were greatest with ST, lowest for SU, and unaffected by RDP. At 24 h of in situ incubation, ruminal disappearance of sorghum silage NDF was increased with decreased RDP, and was greater with ST. At its lowest point, ruminal pH showed a significant interaction of NFCxRDP, with the lowest pH noted with SU and the greater RDP level. These results show that different dietary NFC sources give different production and ruminal responses, and these responses may be altered by the degradability of the dietary protein source fed.